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Monday, October 12, 2015

Fuel Tanks (Part 1)

I'm not a fan of the header tank location in the plans. Between COG shift, quantity constraint, and location, I've elected to build my tanks into my wings instead. I'll be using my outboard wings, because the inboard wings have aileron cables and landing gear in the space I'd want to use. The outboard wings also have dihedral, which will give me a definite lowest point in each tank. This makes it easy for me to locate my sump drain and fuel pickup.

Again, I'm borrowing very heavily from Mark Langford's fuel system, which you can find at his website.

After my wing spars were attached, I was able to begin by cutting slabs of foam for the top and bottom of the wing. I cut templates for the inside of the wing out of 1x4s, glued the 1x4s to the foam, and sanded the foam to the contours. (Note: After doing this for the transfer tank, I decided to do something a bit difference for the other side. See Fuel Tanks (Part 3) for what I think is a better setup.)

I made a bit of an error during my next step. Vinylester is touted as fuel resistant, and therefore, the best fuel tank resin. Unfortunately, not all vinylesters are created equal, and most manufacturers aren't willing to specify if a given vinylester is "fuel safe." As far as I can tell, it's due to the ethanol that winds up in gasoline today. I purchased vinylester from FGCI, went through the process of laying up my fiberglass, and as it was curing I bothered reading the pamphlet that came with it. "Not recommended for fuel." Guess I'll be adding another layer once I get more vinylester, which I ordered from Aircraft Spruce (sold specifically for fuel tanks).

Once the correct vinylester arrived, I began re-doing my layups. At first, I was laying up entire pieces of foam, then cutting them to shape. But I then realized I was wasting a lot of fabric and resin that way due to the dimensions of the foam. I began drawing the shapes I needed onto the foam first, then cutting fabric slightly larger than necessary and laying up. I finished the pieces I needed for my left wing tank, which is only a holding/transfer tank, then began installing them into the wing. My process is as follows:

1. Draw lines ⅜" down from the top of the inside of the spars, and ⅜" up from the bottom
2. Flox and attach front and rear tank walls to front and rear spars, using previously drawn lines
3. Apply duct tape to the tank bottom (to keep from attaching the bottom permanently at this time), then use tape and supports to hold the bottom in place
4. Use flox and fiberglass tape to build a ledge for the tank bottom to attach to
5. Attach tank sides and baffles using flox, taking care not to attach them to the tank bottom
6. Mark location for tank sump
7. Remove tank bottom, secure tank top in place (with duct tape where tank seams will be)
8. Build ledges for tank top as in step 4
9. Remove tank top, apply flox to tank bottom, and secure tank bottom in place permanently
10. Install all lines and fuel level sender
11. Fill with water for a leak test

Assuming the leak test doesn't reveal any catastrophic leaks, I'll fix anything that needs fixing and then install the tank top the same way I installed the tank bottom. The final leak test will be a mild pressure test; at this point if it fails, I'll have to cut open the tank from the top and try to find the failure.

For my fuel and vent lines, I cut ⅜" aluminum plates and drilled and tapped them for AN fittings. Next I cut away the foam where I wanted the lines to pass into the tank and floxed the plates in place.

After I attached the front and rear tank walls to the spars, I got ready to lift the tank bottom in place. I attached five pieces of painter's tape with some slack, thinking they would support the bottom.

As you can kind of see from this picture, five was not enough. However, eleven plus two of my spar-setting jacks was enough. (The funky glare is from four work lights I was using to keep the panels and fresh vinylester warm enough to cure well.) I'm scrambling to finish my fuel tanks before the weather gets too cold.